1. Field of the Invention
The present invention relates to an ophthalmic measurement method, particularly to an ophthalmic measurement method that irradiates the anterior chamber of an eye with a beam of laser light and uses the light scattered by floating protein particles therein to determine protein concentration, composition and the like.
2. Description of the Prior Art
The anterior chamber is located between the cornea and the crystalline lens of the eye, and is filled with aqueous humor. In a normal eye the functioning of the blood-aqueous barrier of the anterior chamber keeps the concentration of albumin, globulin and other such proteins in the aqueous humor at a very low level, and there are no blood cells.
However, when the function of the blood-aqueous barrier is reduced, such as in the aftermath of a white cataract operation, for example, there is an influx of white and red blood cells into the anterior chamber and there is a marked increase in such proteins such as albumin and globulin. For the purpose of post-operative prognoses, it is important to obtain a quantitative measurement of blood cell counts and protein concentrations.
One method of obtaining a quantitative measurement of the protein concentration is to project a laser beam at the protein particles floating in the anterior chamber and measure the intensity of the light scattered by the protein particles.
If the scattering efficiency of the protein particles is A, the concentration of the protein particles in the anterior chamber is N and IO is the intensity of the incident laser beam, then the intensity Is of scattered light from the protein particles will be EQU Is=A.times.N.times.IO (1)
It therefore follows that if the scattering efficiency A and the incident laser beam intensity IO are known beforehand, it is possible to obtain the protein concentration N by measuring scattered light intensity Is.
A system apparatus according to the prior art will now be described with reference to FIG. 2. An optical scanner 2 such as a galvanometer mirror is used to scan a beam of laser light from a laser light source 1 one-dimensionally or two-dimensionally in the aqueous humor space of the anterior chamber where the beam is converged by a lens 4. A control section 3 controls the optical scanner 2 so that the laser beam is guided to the appropriate measurement area in the anterior chamber.
An image of the laser beam in the aqueous humor is formed on a mask 6 by a lens 5, and the image of the scanning laser beam in the aqueous humor of the anterior chamber is also scanned on the mask 6. Scattered light passing through the mask aperture is converted to an electrical signal by a photomultiplier 7, and after the signal has been amplified by an amplifier 8 the intensity of the scattered light is analyzed by an analyzer 9.
When the laser beam image is outside the aperture of the mask 6, the system detects a level of noise from external light and also from the dark current of the photomultiplier, while when the laser beam image is within the mask aperture, the signal component is added onto the noise component, as shown in FIG. 3a. The scattering signal proper can therefore be obtained by subtracting the signal intensity obtained when the laser beam is outside the mask aperture from the signal intensity obtained when the laser beam is within the aperture.
In practice there is not just one but a mixture of a multiplicity of floating protein particles in the aqueous humor of the anterior chamber, and as a result equation (1) is actually expressed as EQU Is=IO.SIGMA.Ak.times.Nk (2)
Here, Ak is the scattering efficiency of protein k and Nk is the concentration of protein k.
As is clear from equation (2), just determining Is will not make it possible to analyze each of the protein components by type. With the composition of each of the multiple protein components being something that is considered to be closely related to the ailment, measuring the composition of each of the multiple protein components in the aqueous humor is of critical significance with respect to diagnosing eye ailments.
One method of determining the composition ratios of protein components involves the application of photon correlation. This technique uses the fact that differences in the diffusion constants of protein components show up as differences in the relaxation times of the scattered light intensity autocorrelation functions, and therefore measures the relaxation times and the weights of the diffusion constants contributing to those relaxation times.
This measurement is usually carried out with the laser beam stationary rather than when it is scanning. However, when using the photon correlation method to obtain the diffusion constant and composition ratio of protein components in the aqueous humor of the anterior chamber, the aqueous humor also contains red and white blood cells with a diameter of around 5 to 20 .mu.m. However, the presence of blood cells in the space scanned by the laser beam gives rise to a signal associated with the scattered light intensity, as shown in FIG. 3b, with the intensity of the scattered light from a blood cell exceeding the intensity of scattered light from protein components.
This type of scattered light degrades the S/N ratio of the autocorrelation function and makes it impossible to achieve an accurate determination of the composition of the protein components.